Typical hybrid electric vehicles (HEVs) use combinations of energy sources to provide motive power to a vehicle. In some serial HEVs, an internal combustion engine is used to power an electric generator, whereby electricity from the generator is then used to power an electric motor that is used to drive the vehicle. The electricity from the generator may also be used to charge an on-board energy storage unit, which can be used to run the electric motor when the engine is off, or in some cases, to assist in running the motor in tandem with the engine. Such HEVs frequently have regenerative braking systems, which running the motor as an electric generator during braking to provide braking torque and recapture propulsion energy. In addition, such HEVs will typically transmit propulsion energy between the generator, the electric motor, and the energy storage across a common DC propulsion bus.
Of particular interest here are heavy duty HEVs. A heavy duty vehicle is legally defined as being over 8,500 lbs. Heavy duty HEVs (as used herein) are typically over 10,000 lbs, and may include metropolitan transit buses, semi tractor trailers, refuse collection and/or other heavy duty vocational trucks, etc. Moreover, unlike stationary hybrid systems and even automotive HEVs, heavy duty HEVs face unique challenges associated with their substantially higher power levels, the mobile environment under substantially higher loads, increased safety requirements, and increased performance and duty cycle requirements.
The choice and design of the HEVs propulsion energy storage unit is a key parameter for the HEV drive system's overall performance. While various energy storage technologies are available, the choice typically represents compromises between desired power density and desired energy density. For example, energy-type energy storage devices, such as lead-acid or lithium-ion rechargeable batteries have low power densities on the order of 100 W/kg, but have high energy densities on the order of 100 Wh/kg. Power-type energy storage devices, such as ultracapacitors, on the other hand, have high power densities on the order of 5,000 W/kg, but have low energy densities on the order of 10 Wh/kg. Accordingly, ultracapacitors are able to provide a great deal of power over a short period of time, while batteries are able to provide a modest amount of power over a relatively long period of time. The rechargability of these energy storage modules substantially mirrors these characteristics.
An ultracapacitor is typically able to receive and store a great deal of power over a short period of time, while a rechargeable battery must be presented with smaller amounts of power over longer periods of time. Accordingly, during a regenerative braking period, an ultracapacitor may be able to store all the electricity produced during one braking period, but this might use up all the ultracapacitors capacity; while a battery may be able to receive charge during many braking periods, but may not be able to receive the entire amount of electricity produced during one braking period. Typically, regardless of the choice of technology, the energy storage unit will often need to be oversized to meet all of the vehicle's performance requirements.
Since the state of charge (SOC) of the on-board energy storage module necessarily fluctuates during driving, it is common for the energy storage to include a DC/DC converter between itself and the DC propulsion for power to flow as needed. As a practical matter though, the DC/DC converter may face unique challenges and be limited in its performance and capacity to buck or boost voltages. In particular, under driving conditions it may be necessary to reliably boost relatively large currents (e.g., 300 A) in a mobile environment. With the high voltages encountered HEVs, a high power, insulated-gate bipolar transistor (“IGBT”)-based DC/DC converter may be required. However, IGBT (or similar) DC/DC converters may be have certain drawbacks in this application. In particular, with DC/DC converters, the greater the ratio that the voltage is bucked or boosted, the greater the need for fast switching. However, IGBT's are limited in their switching frequency (e.g., 20 kHz). As a result of this limitation voltage ripple is increased at the higher frequencies and power quality is reduced. In addition, IGBT's emit heat at high switching frequencies and become less efficient. Moreover, at the power levels associated with vehicle propulsion thermal management may be required.
In a heavy duty hybrid application, these challenges may more pronounced. For example, in a heavy duty HEV having a battery-based energy storage, a DC/DC converter may need to convert voltage across a step from a low SOC of 200 VDC to a high SOC of 700 VDC; and where the HEV has an ultracapacitor-based energy storage instead, the DC/DC may need to boost voltage from a nominal (˜0 VDC) voltage up to 700 VDC. Thus, in either type of HEV, at low energy storage SOC, the voltage step is substantial, and undesirable losses and noise are introduced. To mitigate some of these deficiencies active cooling, electronic noise reduction, etc. may be used however. Thus, in current hybrid drive systems, additional and/or high performance equipment, which is often expensive, may be required to meet these high voltage ratio and propulsion power requirements.